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Register a new userUltrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry
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Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime.
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The dynamics of the photoinduced commensurate to incommensurate charge density wave (CDW) phase transition in 4Hb-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime the CDW reforms on a 150 ps timescale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T-layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation the phase transition is achieved showing a second order character in contrast to the first order behavior in thermal equilibrium.
Measuring terahertz (THz) conductivity on an ultrafast time scale is an excellent way to observe charge-carrier dynamics in semiconductors as a function of time after photoexcitation. However, a conductivity measurement alone cannot separate the effects of charge-carrier recombination from effective mass changes as charges cool and experience different regions of the electronic band structure. Here we present a form of time-resolved magneto-THz spectroscopy which allows us to measure cyclotron effective mass on a picosecond time scale. We demonstrate this technique by observing electron cooling in the technologically-significant narrow-bandgap semiconductor indium antimonide (InSb). A significant reduction of electron effective mass from 0.032$m_mathrm{e}$ to 0.017$m_mathrm{e}$ is observed in the first 200ps after injecting hot electrons. Measurement of electron effective mass in InSb as a function of photo-injected electron density agrees well with conduction band non-parabolicity predictions from ab initio calculations of the quasiparticle band structure.
The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS$_2$ and WS$_2$ on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9-2.3 eV between our systems. The transient conduction band signals decay on a sub-100 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe two timescales on the order of 200 fs and 50 ps, respectively, for the conduction band decay in MoS$_2$. These multiple timescales are explained by Auger recombination involving MoS$_2$ and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.
We apply ultrafast X-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO$_3$ after above-bandgap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO$_3$: the relevant excited charge carriers must remain localized to be consistent with the data.
The zone-center $E_{2g}$ modes play a crucial role in MgB$_2$, controlling the scattering mechanisms in the normal state as well the superconducting pairing. Here, we demonstrate via first-principles quantum-field theory calculations that, due to the anisotropic electron-phonon interaction, a $hot$-$phonon$ regime where the $E_{2g}$ phonons can achieve significantly larger effective populations than other modes, is triggered in MgB$_2$ by the interaction with an ultra-short laser pulse. Spectral signatures of this scenario in ultrafast pump-probe Raman spectroscopy are discussed in detail, revealing also a fundamental role of nonadiabatic processes in the optical features of the $E_{2g}$ mode.
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